This invention relates generally to methods and apparatus for Computed Tomography (CT) imaging and other radiation imaging systems and, more particularly, to facilitating a reduction of artifacts in reconstructed images.
In at least some CT imaging system configurations, an x-ray source projects a fan-shaped x-ray beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated x-ray beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the x-ray intensity signal, facilitating computation of the beam attenuation, at the detector location. The intensity measurements from all of the detectors are acquired separately and are used to compute a profile of the line integral of the linear attenuation coefficient of the object, which is denoted as projection data. Volumetric CT imaging systems have source collimation such that a cone-shaped beam of x-rays illuminate the patient to be imaged and an area detector is used to measure the x-ray energy that is not attenuated by the patient, giving rise to a two-dimensional projection image.
In at least some known “third generation” CT systems, the relative orientation of the x-ray source and the detector array are held fixed. The x-ray source and the detector array are then rotated with a gantry within the imaging plane, and around the object to be imaged, so the angle at which the x-ray beam intersects the object changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam from a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams and removing scattered x-rays (x-rays that interact with the patient being imaged and are redirected towards the detector) received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator.
In particular embodiments of a volumetric CT system, area radiation detector arrays may be approximately twenty centimeters (cm) square or less and the array and gantry are rotated 360° about the patient to produce a complete image. Conversely, the x-ray tube and gantry can be held constant (stationary gantry) while the object is rotated during data acquisition. These schemes can be implemented in industrial CT systems, such as, for example, but not limited to, a baggage scanning CT system for an airport or other transportation center. The former CT system topology will be described in detail in the text that follows. However, methods described herein are equally applicable to stationary gantry systems and are not meant to limit the scope of the invention. Additionally, in the text that follows, the term “detector sections” refers to both linear radiation detectors and area radiation detectors.
Although the collection of projection images as proposed above is not mathematically complete when the embodiment includes area radiation detectors, it is possible to reconstruct a volumetric or three-dimensional (3-D) representation of the object from the measured data. Moreover, to acquire enough data for diagnostic image quality, the shadow of the patient on the detector, resulting from x-ray illumination of the patient, must not extend past the edge of the detector. Considering typical magnification in a CT system and a particular embodiment of a CT system utilizing a 20-cm square area detector, this requires that the diameter of the patient be approximately 13 cm or less. One known solution to increase the field of view of the imaging system involves using two or more digital radiation detector arrays that are butted together. However, such detector arrays generally have areas or zones in which the x-rays are not detected in the region where the detectors butt. These zones are commonly referred to as dead zones. Because x-ray projection data is not acquired in the dead zones, the missing projection data caused by the dead zones may produce artifacts in the reconstructed images of the 3-D volume of the scanned object.